[time-nuts] ADEV vs MDEV
lists at cq.nu
Sat Feb 6 20:43:51 UTC 2010
Unfortunately, the most common approach is to simply say "it's a wide band loop, no need to check it ...."
On Feb 6, 2010, at 3:30 PM, Bruce Griffiths wrote:
> To a first approximation injection locking alters the loop parameters so its important to measure the actual PLL characteristics with the loop closed and not just use the PLL parameters inferred from the OCXO EFC transfer function etc.
> The noise of the OCXO used as a VCXO will limit the noise floor.
> An ADEV noise floor of 1E-13 isnt likely when using an HP10811A as the VCXO for example.
> Bob Camp wrote:
>> It's possible / likely to injection lock with the tight loop approach and get data that's much better than reality. A lot depends on the specific oscillators under test and the buffers (if any) between the oscillators and mixer.
>> If your OCVCXO has a tuning slope of 0.1 ppm / volt then a part in 10^14 is going to be at the 100 of nanovolts level. Certainly not impossible, but it does present it's own set of issues. Lab gear to do it is available, but not all that common. DC offsets and their temperature coefficients along with thermocouple effects could make things exciting.
>> There is no perfect way to do any of this, only a lot of compromises here or there. Each approach has stuff you need to watch out for.
>> From: "WarrenS" <warrensjmail-one at yahoo.com>
>> Sent: Saturday, February 06, 2010 2:19 PM
>> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
>> Subject: Re: [time-nuts] ADEV vs MDEV
>>> Peat said:
>>>> I would appreciate any comments or observations on the topic of apparatus with demonstrated stability measurements.
>>>> My motivation is to discover the SIMPLEST scheme for making stability measurements at the 1E-13 in 1s performance level.
>>> If you accept that the measurement is going to limited by the Reference Osc,
>>> for Low COST and SIMPLE, with the ability to measure ADEVs at that level,
>>> Can't beat a simple analog version of NIST's "Tight Phase-Lock Loop Method of measuring Freq stability".
>>> http://tf.nist.gov/phase/Properties/one.htm#oneone Fig 1.7
>>> By replacing the "Voltage to freq converter, Freq counter & Printer with a Radio shack type PC data logging DVM,
>>> It can be up and running from scratch in under an Hr, with no high end test equipment needed.
>>> If you want performance that exceeds the best of most DMTD at low Tau it takes a little more work
>>> and a higher speed oversampling ADC data logger and a good offset voltage.
>>> I must add this is not a popular solution (Or a general Purpose one) but
>>> IF you know analog and have a GOOD osc with EFC to use for the reference,
>>> as far as I've been able to determine it is the BEST SIMPLE answer that allows High performance.
>>> Limited by My HP10811 Ref OSC, I'm getting better than 1e-12 in 0.1 sec (at 30 Hz Bandwidth)
>>> Basic modified NIST Block Diag attached:
>>> The NIST paper sums it up quite nicely:
>>> 'It is not difficult to achieve a sensitivity of a part in e14 per Hz resolution
>>> so one has excellent precision capabilities with this system.'
>>> This does not address your other question of ADEV vs MDEV,
>>> What I've described is just a simple way to get the Low cost, GOOD Raw data.
>>> What you then do with that Data is a different subject.
>>> You can run the raw data thru one of the many ADEV programs out there, 'Plotter' being my choice.
>>> Have fun
>>> [time-nuts] ADEV vs MDEV
>>> Pete Rawson peterawson at earthlink.net
>>> Sat Feb 6 03:59:18 UTC 2010
>>> Efforts are underway to develop a low cost DMTD apparatus with
>>> demonstrated stability measurements of 1E-13 in 1s. It seems that
>>> existing TI counters can reach this goal in 10s. (using MDEV estimate
>>> or 100+s. using ADEV estimate). The question is; does the MDEV tool
>>> provide an appropriate measure of stability in this time range, or is
>>> the ADEV estimate a more correct answer?
>>> The TI performance I'm referring to is the 20-25 ps, single shot TI,
>>> typical for theHP5370A/B, the SR620 or the CNT81/91. I have data
>>> from my CNT81showing MDEV < 1E-13 in 10s. and I believe the
>>> other counters behave similarly.
>>> I would appreciate any comments or observations on this topic.
>>> My motivation is to discover the simplest scheme for making
>>> stability measurements at this performance level; this is NOT
>>> even close to the state-of-the-art, but can still be useful.
>>> Pete Rawson
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